While most endorsements are overrated, the endorsement of Pat Robertson for Rudy Giuliani is HUGE for his chances of earning the Republican nomination. It will help to shore up his weak right flank due to social issues. I'm sure there will still be many social conservatives who will not support him in the primary but will now be less apprehensive of supporting him in the general.
An example of an endorsement that carries very little weight is Brownback's endorsement of McCain. McCain's pro-life credentials were never in question, so this does nothing more than restate the obvious that McCain is the most pro-life candidate of the top-tier candidates.